Why bcrypt.compare function no need to use salt, when compare with origin password?

[xgqfrms]

Why bcrypt.compare function no need to use salt, when compare with origin password?

hash-salt-test.js

const bcrypt = require('bcrypt');

const hash = async function (password, salt = 10) {
  // ❓default salt: 10
  console.log(`❓salt =`, salt);
  const hashed = await bcrypt.hash(password, salt);
  return hashed;
}

const check = async function (password, hashed) {
  // ❓why no need to use any salt when call compare function
  const result = await bcrypt.compare(password, hashed);
  return result;
}

const test = async function (salt = 10) {
  // password length is 10 ✅, less than 72 bytes ❓
  const pwd = 'pwd1234567';
  const hashed = await hash(pwd, salt);
  const result = await check(pwd, hashed);
  console.log(`\nhash with salt`, hashed);
  console.log(`result =`, result);
}

test();
test(5);
test(11);
test(15);
test(17);

/*

 $ node ./hash-salt-test.js

❓salt = 10
❓salt = 5
❓salt = 11
❓salt = 15
❓salt = 17

hash with salt $2b$05$.zICWxt0wAjGb58kIN6GwekMtP5jUeYD34/wqfE17yYqZ3rO43RV.
result = true

hash with salt $2b$10$PPJdjtaboGieRgk1p86bsuCHIqmiFhqaRGwHrpVs5/dZ3ZLC8b33m
result = true

hash with salt $2b$11$Y.Ozy9S5gtZUR1XD3egrGeZ1H8HmPOUqBDochpGM5pLCIP2tk38Xa
result = true

hash with salt $2b$15$UBGFkZRAxGgMMTaRDmxJ1eTgCBM.ovvESEyJJwrGeIpk/Vw3NPFkC
result = true

hash with salt $2b$17$zyjemy4KDsQDO8Q2kCgYeeW/FsG3kiWp0hXl4t.o8GUWrWjA4.R.S
result = true

*/

[xgqfrms]

#963 (comment)

[xgqfrms]

Per bcrypt implementation, only the first 72 bytes of a string are used.
Any extra bytes are ignored when matching passwords.

Note that this is not the first 72 characters.
It is possible for a string to contain less than 72 characters, while taking up more than 72 bytes
(e.g. a UTF-8 encoded string containing emojis).

#935 (comment)

[recrsn]

Bcrypt generates the hashed password in modular crypt format (MCF) the hash itself contains information about the algorithm, rounds and the salt